David Shayler, the former MI5 officer, wanted in Britain for allegedly breaching the Official Secrets Act, is returning home voluntarily on Monday 21 August, 2000, after three years in exile.

He fled to France after revealing information gathered during his five years working for the Security Service (commonly known as MI5). The government tried, unsuccessfully to extradite him from France.

The court there ruled that his offences were of a political nature and therefore exempted him from extradition. Legal affairs Correspondent Jane Peel explains what he will face on his return.

Why is he coming home now?

He's had enough of living in exile.

On 2 October the Human Rights Act comes into force. Shayler believes he can use this legislation - which will be in force long before any trial takes place - as a defence.

He believes the Official Secrets Act 1989 under which he'll be charged is incompatible with the Human Rights Act.

Why? The Act guarantees a right to freedom of expression. This right can be restricted on grounds of national security, but only if the restriction is necessary, and the restriction must be "proportionate".

There is an argument that the Official Secrets Act falls foul of that because Shayler will not be able to argue that he caused no damage by his revelations or that he made them in the public interest.

Those arguments are no defence to charges under the OSA. The fact that he disclosed information gleaned through his secret work is enough to convict him.

The football season has just started. David Shayler is a fanatical Middlesbrough fan and has missed watching them play. We thought he was going to come home on Wednesday 23rd - exactly three years since he left the UK. It might be significant that Middlesbrough have a home match against Spurs on Tuesday.

What will happen to him now?

He will be met on arrival at Dover by officers of Scotland Yard's Special Branch, arrested and taken to London. If this happens it's expected he'll be charged with two offences under the Official Secrets Act relating to disclosures of information and documents he made to the Mail on Sunday Newspaper in August 1997. It's then posssible he will be bailed to appear in court later in the week.

Is he likely to remain on bail while awaiting trial?

Yes. The Crown Prosecution Service have privately given him an undertaking that they will not oppose bail. This is because he is returning to the UK voluntarily so he is unlikely to abscond. Also, he has already spent three and a half months in custody in prison in France while he was awaiting possible extradition. This would be taken off any prison sentence imposed anyway.

What exactly is the crime Shayler is alleged to have committed?

Under section 1 of the 1989 Official Secrets Act it is an offence for any serving or former member of the security or intelligence services (MI5 and MI6) without lawful authority to disclose any information, document or other article which relates to security or intelligence which is or has been in his possession by virtue of his position as a member of any of those services or in the course of his work.

What is the penalty on conviction?

A maximum of two years' imprisonment for each offence if the case is tried in the Crown Court.

How much time would David Shayler serve if convicted?

If he were to get the maximum penalty of two years, he would be expected to serve half of that time in custody. However, because he has spent some time in custody already, he would probably serve about eight and a half months.

He would spend longer in prison if he was sentenced to more than one term of two years (for example if he received two years each for two offences and the judge ordered that they run consecutively rather than concurrently).

What information has he disclosed?

His initial revelations in the Mail on Sunday three years ago included the fact that MI5 had bugged prominent individuals, and kept files on leading politicians, including Peter Mandelson, the Home Secretary Jack Straw and the former Prime Minister, Sir Edward Heath.

He complained of bureaucracy and inefficiency in MI5 and blamed the Security Service for failing to put an end to the IRA's bombing campaign in Britain.

Since then there've been further revelations. He claims MI6 knew of and helped fund a failed plot to assassinate the Libyan leader, Colonel Gaddafi and that about £100,000 of taxpayers' money was spent on this. He said MI5 had advance intelligence about the plan to bomb London's Israeli Embassy, but failed to prevent it.

Most recently he wrote a detailed article in Punch magazine giving more details about what he says was the failure of MI5 to put an end to IRA activities in Britain before their ceasefire.

What has been the official response, if any, to these allegations?

Gaddafi plot: The Foreign Secretary, Robin Cook said: "There was no government-inspired plan to assassinate Gaddafi. There was no SIS (MI6) proposal to do it, and I'm fairly clear there has never been any SIS involvement. I'm perfectly clear that these allegations have no basis in fact. It is pure fantasy."

Israeli Embassy bombing: The Home Secretary, Jack Straw said: "It is not the case that such information as the Security Service had in their possession would have enabled it to prevent the Israeli Embassy bombing from happening."

How damaging is this information to national security?

That depends who you ask. David Shayler says he has been careful to avoid revealing any information that might put national security or individuals at risk.

He points out that a Special Branch assessment made for the French extradition proceedings said "these disclosures would not cause serious damage to the work of the Security Service or national security".

But that assessment also said some of the information he gave to the Mail on Sunday was "extremely sensitive... had the potential to cause serious harm to the work of the Security Services and grave damage to national security".

This is certainly the view taken by the government and MI5. There is particular concern that he has revealed details of security operations.

Why does Shayler say he did it?

He says while at MI5 he became disillusioned about its management, inefficiencies and bureaucracy. But he says he left after innocent civilians were murdered when the MI6-backed plot to kill Gaddafi went wrong - he says he didn't want to be a part of a system which was involved in and condoned murder.

He says he had a duty to inform the public of these matters. He says he should be able to criticise the state without fear of persecution.